This invention relates to illumination of a silicon wafer when using a machine vision system to obtain alignment characteristics of the wafer.
Silicon wafer fabrication is generally described as a series of sequential photo-chemical processing steps that create an array of semiconductor devices. The silicon wafer, the substrate upon which the semiconductor devices are fabricated, is a flat single monocrystal of silicon. It is typically in the shape of a circle of diameter 150 mm, 200 mm or 300 mm. The various wafer fabrication processing steps require precise alignment of the silicon wafer. Precise alignment may be required for a particular processing step because that step depends on precise crystal alignment either for manufacturing efficacy or manufacturing repeatability such as an ion beam deposition step. Alternatively, precise alignment may be required because a particular processing step is pattern-dependent and needs to be photographically registered with one of the previous steps such as a photolithography step.
In order to facilitate this precise alignment, wafers are manufactured to agreed upon standards with specific features. For example, SEMI M1-0305 Specifications for Polished Monocrystalline Silicon Wafers defines some of these standards and describes notches or flats cut into the outside perimeter of the wafer permitting wafer orientation to be determined by examination of the wafer perimeter.
Wafer Prealignment (sometimes called Wafer Coarse Alignment) is an automated process of examining the shape of a silicon wafer and its notches, flats or other geometric shape characteristics to determine the alignment of a wafer. Alignment of the wafer means determining the position and orientation of the wafer relative to a particular coordinate system. The physical positional accuracy of such an alignment could range from a fraction of a micron to a few millimeters in position and from a few thousands of a degree to a degree or two in orientation. It is possible for some of the processing steps described above to involve creating fiducial marks on the surface of the wafer. In later processing steps, those newly created fiducial marks can also be used for wafer alignment. Such a wafer alignment step that uses fiducial marks on the surface of the wafer is called a fine alignment step and is not the subject of this application. However, it is important to note that even when a fine alignment step is performed to align a wafer, a coarse alignment step is typically performed first in order to reduce the search area of the fiducial mark.
Conventional wafer alignment systems and methods employ LED illumination that provides backlight illumination of the wafer. To provide the requisite uniform diffuse illumination, the LED illuminators require a diffuser that distributes the point source illumination of the LED into a wide area diffused mode of illumination proximate to the peripheral region of the wafer. While effective, the conventional illumination systems and methods become increasingly complex and expensive as the wafer fabrication industry continues in its trend toward larger wafer sizes.
Accordingly, there is a need for a low cost method and apparatus for providing uniform and diffuse illumination over a wide area for backlighting wafers during coarse alignment.